A comprehensive understanding of the mechanisms associated with the generation and modulation of immunological T cell memory will lead to a better understanding of how the immune system controls viral infections but also causes immune-mediated pathology. Our studies on cross-reactive T cell responses during Epstein Barr virus (EBV)infection only begin to scratch the surface of the prevalence and potential impact of cross-reactive T cell responses on both vaccine development and immunopathology. There is very little understanding of the structural and functional interaction of one TCR with two different ligands. We have identified directly ex vivo and in bulk T cell cultures HLA-A2-restricted cross-reactive CDS T cell responses that recognize both EBV BMLF-1 and influenza A M1 HLA-A2 restricted epitopes. These cross-reactive T cells were found to participate in acute infectious mononucleosis (IM). Five of 8 young adult HLA-A2+ patients experiencing IM had an increased number of influenza virus (FLU)-M158-66 specific CD8+ T cells in their peripheral blood as compared to healthy donors. Two of 5 IM patients with augmented FLU-M1 responses had high levels of tetramer-defined cross-reactive cells as measured directly ex vivo in their peripheral blood. EBV likely activates multiple populations of cross-reactive memory cells involved in the development of IM, and we have been able to provide evidence that those specific to FLU-M1 can contribute to this phenomenon. In order to better understand how cross-reactive CDS T cells may be modulating disease outcome by enhancing viral clearance or inducing immunopathology, such as that seen in IM, we will characterize the cross-reactive TCR, both functionally and structurally, and examine how cross-reactivity influences the evolution of antigen-specific TCR repertoire development and disease outcome in humans. Aim 1. Correlate disease outcome during acute infectious mononucleosis (IM) with activation of cross- reactive CDS T cells. Aim 2. Examine the structural and functional interaction of a cross-reactive TCR that recognizes both EBV BMLF-1 and FLU M1 epitopes. Aim 3. Determine the effect of heterologous virus infections on TCR repertoire development and stability of memory CDS T cells during human viral infections. Aim 4. Determine the level of cross-reactive CDS T cell responses in the young and the elderly. These studies will examine how an individual's history of viral infections can potentially influence the immune response and ultimate outcome whether good or bad to each new infection.